Wireless systems in the cellular context are currently being implemented using fourth generation (4G) standards. These 4G standards include Long Term Evolution (LTE) standards developed by the 3G Partnership Project (3GPP). LTE cellular systems make use of an Internet protocol (IP) based packet core referred to as Evolved Packet Core (EPC). The EPC interconnects multiple base stations within the system. A given base station, also referred to as an evolved Node B (eNB), communicates over an air interface with multiple user terminals. Individual user terminals are also referred to as user equipment (UE).
The air interface between an eNB and UE in an LTE cellular system includes a variety of uplink and downlink channels. See, for example, 3GPP TS 36.211, V9.1.0, 3rd Generation Partnership Project Technical Specification, Group Radio Access Network, Evolved Universal Terrestrial Radio Access (E-UTRA), Physical Channels and Modulation (Release 9), March 2010, which is incorporated by reference herein.
In LTE systems, certain uplink or downlink channels may make use of single-carrier quadrature modulation, such as quadrature phase-shift keyed (QPSK), in which information is conveyed in symbols each comprising one of four distinct phase values, or quadrature amplitude modulation (QAM), in which information is conveyed in symbols each comprising a distinct combination of a phase value and an amplitude value.
Techniques are known in the art for generating various types of modulation signals using direct digital synthesis (DDS). See, for example, PCT International Publication No. WO 99/25104 and U.S. Pat. No. 6,002,923.
However, these conventional techniques can be problematic, particularly in the case of quadrature modulation. For example, techniques such as those described in the above-cited references typically rely on the use of frequency multipliers, which can cause a substantial amount of phase uncertainty in the resulting output signal. Even if the frequency multiplication process begins with a signal having a highly accurate phase, multiplication to increase the frequency will increase the phase variation as well, while also introducing additional phase noise. Moreover, output filtering that is typically used to select a particular multiplied frequency component will introduce additional phase variation as a function of frequency across the filter bandwidth.
The phase uncertainty attributable to use of frequency multiplication in conventional techniques can cause quadrature symbols to rotate into the space of adjacent symbols, leading to sampling errors and a significant reduction in achievable bit error rate (HER) across the output frequency band.
An alternative to use of frequency multipliers is to configure the direct digital synthesis to operate at the desired output frequency. However, this approach is impractical in applications involving high frequencies. In addition, such an approach generally requires the use of separate in-phase (I) and quadrature-phase (Q) channels, each requiring separate components such as mixers, filters and analog-to-digital converters.